The present invention relates to a thermocontrol method for an injection molding machine, more precisely relates to a method for controlling temperature of thermocontrolled components, e.g. an injection cylinder, of the injection molding-machine to correspond to the instant operating status thereof.
Conventionally, PID (Proportional-Integral-Differential) control method is usually adopted to control the temperature of thermocontrolled components, e.g. an injection cylinder, of an injection molding machine.
The PID control method is based on a proportional action (P), which is in proportion to a control deviation; an integral action (I), which is based on an integrated value of the control deviation; and a differential action (D), which is based on differential coefficients of the thermocontrolled components. Using the PID control method, the object temperature can be maintained when the thermocontrolled components are under certain stable conditions.
However, the injection molding machine has various operating states such as stop, temperature rise, mold, pause, etc.. Furthermore, heating elements and cooling elements are different devices in each status. For example, in the temperature rise state, radiant heat is the main cooling element. On the other hand, heat from the heater, the heat of friction caused by an injection screw acting upon the resin, etc. are the heating elements in the mold status; while the natural radiant heat, the endothermic reaction by resin supplied, etc. are the cooling elements therein.
Therefore, in conventional PID control, the temperature of an injection cylinder, for example, may be higher (overshoot, P.sub.o) or may be lower (undershoot, P.sub.u) than the object temperature, as shown in FIG. 5: A Temperature Graph of the Injection Cylinder. The overshoot P.sub.o and undershoot P.sub.u of the injection cylinder have a large effect on the viscosity of molten resin, so that they may become factors in inferior production quality. Moreover, the overshoot P.sub.o causes resin deterioration when the object temperature is exceeded and the resin deterioration temperature is attained, resulting in inferior products. To avoid inferior production quality, manual control of the temperature of the injection cylinder, based on the experience of a skilled operator, is required.